Magnetic particles can be used for separating a specific substance from a liquid medium. Such particles are coated with a specific reagent interacting with the substance or the substance intrinsically adheres to the surface of the particles. The particles together with the substance attached to them are separated from the liquid. This has conventionally been done by removing the liquid. Nowadays also magnetic separating probes are used. Such a probe comprises a magnet moveable up and down within an elongate covering shield. The particles are collected on the surface of the shield keeping the magnet in its lower position. The probe is then transferred to another liquid, and the magnet is moved to its upper position, whereby the particles are released. Such a process has been described e.g. in U.S. Pat. No. 6,040,192 and U.S. Pat. No. 6,447,729.
The concentration ratio, i.e. the ratio of the start volume to the end volume, is preferably maximized so that collected particles can be further processed as effectively as possible. The start volume is often relatively big (e.g. >1000 μl), and the concentration of the substance to be collected may be very low. The collected particles may be released into a very small liquid volume by utilizing liquid adhesion, whereby the particles are washed off by a small liquid amount that wets the surface on which the particles have been collected. With a suitable design of the probe and vessel, very small release volumes (e.g. 5-10 μl) may be achieved. Such processes have been described e.g. in U.S. Pat. No. 6,207,463, U.S. Pat. No. 6,448,092, and U.S. Pat. No. 6,596,162.
WO 99/40444, US 2006269385, and WO 2009076560 describe processes, in which magnets are additionally used under the receiving vessels in order to facilitate the release of particles from the probes into the liquid.